// 定义一个mixin用于生成margin样式
@mixin margin($top: null, $right: null, $bottom: null, $left: null) {
	@if $top != null {
		margin-top: $top;
	}
	@if $right != null {
		margin-right: $right;
	}
	@if $bottom != null {
		margin-bottom: $bottom;
	}
	@if $left != null {
		margin-left: $left;
	}
}

// 定义一个mixin用于生成padding样式
@mixin padding($top: null, $right: null, $bottom: null, $left: null) {
	@if $top != null {
		padding-top: $top;
	}
	@if $right != null {
		padding-right: $right;
	}
	@if $bottom != null {
		padding-bottom: $bottom;
	}
	@if $left != null {
		padding-left: $left;
	}
}
